The greater bandwidth and low transmission loss of optical fibers coupled with the capabilities of optical devices has led to the evolution of high data transmission in the telecommunications industry. In general, these systems have relied on FM subcarrier modulation to effect a digital-to-analog-to-digital conversion to effect the communication of digital signals. However, the large bandwidths required as well as the required conversion from analog to digital signals has restricted the use of these systems in many low cost requirement systems. One such industry that has required multicarrier communication is the CATV and other video distribution industries, which rely primarily on amplitude modulation vestigial side band (AM-VSB) signal transmission. These industries would benefit from a transmission and reception system that is highly linear and not reduced greatly in performance by ill effects of noise. To be sure, the CATV industry requires carrier-to-noise ratios on the order of nearly 50 dB, substantially greater than that required for FM systems. Furthermore, signal distortion across the entire system must have cumulative power on the order of magnitude of -50 dBc. While lasers have been developed to meet these requirements, it is essential that improvements be made in the entire system to effect the required signal transmission characteristics. The present invention is related to a low cost photodetector that supplies the requirements of linearity, low noise and low distortion needed for CATV fiber-to-the-home (FTTH) systems.
The basic performance of a PIN photodetector is described presently. Semiconductor pn junctions are employed widely for photodetection. The physics of their use in this application is as follows. Turning to FIG. 1, we see the energy band diagram of a pn junction used as a photodetector. Light absorbed at the p region of the junction, creating an electron-hole pair as shown. If the absorption of the light occurs at a point of the p-side that is within a diffusion length (the average length that a minority carrier will traverse before recombining with an opposite carrier) of the depletion region, the electron will in all probability reach the boundary layer and drift across the depletion layer. Such an electron will then contribute a charge e to the external circuit, thereby giving an electrical indication of the optical signal absorbed by the junction photodiode. Should the light be within the absorption band of the detector and be received on the n-side of the depletion region of the junction, another electron-hole pair will be created, and the hole will traverse to the junction again be diffusion, and then drift across the junction. Again, this will result in a charge flow e across the external load. Alternatively, and preferably, the photon could be absorbed in the depletion region, and the electron-hole pair created results therein. The electron and hole created will drift in opposite directions under the field of the bias potential. In this arrangement, each carrier will traverse a length that is less than the junction width and the contribution to the charge flow in an external circuit is e as determined from basic transport equations. This method is most desirable, since each absorption gives rise to a charge of magnitude e, and the delay in current response time due to finite diffusion time is avoided. From this observation comes the structure having a layer of intrinsic (i) semiconductor sandwiched between the p and n layer, thus the PIN diode. The intrinsic layer is a high resistivity layer and the potential drop of the bias potential is greatest across the intrinsic layer. Furthermore, the intrinsic layer is generally made large enough to assure that most incident photons are absorbed within this layer.
Turning to FIG. 2a, we see a cross sectional view of a conventional PIN photodetector. The intrinsic absorption layer consists of InGaAs ternary material which is epitaxially grown lattice matched on a semiconductor substrate. The substrate is generally chosen to be transparent in the wavelength range desired to be detected, and in the case of an InGaAs absorption layer, an n.sup.+ InP substrate is chosen as it is transparent in the range 1.3-1.55 microns in wavelength. Under operating conditions, the intrinsic layer is depleted fully by a top pn junction. The PIN structure can be achieved by simply growing a layer of p.sup.+ InGaAs or InP over a layer of intrinsic InGaAs (not shown), but in most practical devices, fabrication is effected by having a localized p.sup.+ region 201 formed by diffusion of a suitable dopant, for example Zn, into a layer of InP through a suitable mask, for example SiN.sub.x grown on the InP top layer. The desired effect of this practical technique of fabrication is a planar structure, with a well defined junction area (by virtue of the mask diffusion technique) and minimum surface current leakage by virtue of the buried junction. A PIN photodetector of this structure can be illuminated in the near infra-red either from the top through the pn junction or from the rear through the transparent InP substrate.
The device operates under the condition of reverse bias to effect the desired field direction to facilitate carrier flow upon absorption of light of the proper wavelength. The reverse bias potential of a few volts is usually enough to amply deplete the intrinsic layer of carriers, and in the absence of light signals, only a small reverse current flows across the boundaries. Finally, it is important to recognize that due the absence of a gain mechanism in the PIN diode, the gain-bandwidth product is nearly equal to the bandwidth itself, the bandwidth determined by the transit time of electron-hole pairs, and accordingly by the thickness of the intrinsic layer. Accordingly, the thickness of the intrinsic layer's effect on absorption efficiency must be balanced against its ill-effect on time of transit. In reality, the bandwidth of the PIN detector is limited by factors such as time constants of resistance and capacitance of the device, and bandwidths on the order of GHz are achievable.
The speed of response of a detector with low mobility in the contact layer will be highly position dependent. By position dependence it is meant the position of the incidence of the input radiation. For example, turning to FIG. 2b, we see the cross section of a PIN detector that has an asymmetric electrical contact 201 (as opposed to a symmetric annular contact) and a layer of InP, that exhibits low carrier mobility, in which is diffused a p.sup.+ region. Radiation that is incident upon the photosensitive layer creates carriers that traverse the p.sup.+ region to the contact 201 thereby effecting an electrical signal in the external circuit. Clearly photogenerated carriers that must traverse a large distance in low mobility material will take a substantially larger amount of time than carriers that must traverse only a short distance in the low mobility material. By comparison, for high mobility material, the difference in transit time in the dimensions of the device is negligible. Therefore, for detectors with a low mobility contact layer (such as the InP layer as shown in FIG. 2a), the speed of response of the detector will depend on the distance that a photogenerated carrier will have to travel to reach the contact metallization. For example, a carrier generated at point A of FIG. 2a will traverse to the contact in a much shorter time than one generated at point B. This effect is further exacerbated when the contact metallization is asymmetric to the region where the carriers are generated. In the extreme case where the contact metallization is restricted to a small partial circumferential region of this area, the positional dependence is extreme. (This is the case shown in FIG. 2a). Carriers generated in close proximity to the contact metallization have a short distance to travel and thus a fast speed of response. However, carriers generated in a position diametrically opposite to the contact metallization, have the longest distance to travel, and hence result in the longest response time. This positional dependence has been well documented by the applicants in measurements with InP cap layers and asymmetric contacts. The difference in the speed of response of the device, depending on the location of the photogenerated carriers results in undesirable distortion. The distortion results for example where two signals arrive at different times, that which arrives first may impinge the detector at a position farther than that which arrives later. Because of the traversal time lags, these signals could interfere, and thereby intermodulation distortion results. Another example is where the area of the incident signal is impingent on a relatively large area of the detector. The portion of the incident beam farthest from the contact metallization will generate carriers that are farther from those generated from the signal closer to the contact, and the signal is distorted. In the case of the low mobility InP contact layer even replacing the metallization contact with a contact that is symmetric to the region where the carriers are generated does not cure the problem (this configuration is shown in FIG. 2c). Because the InP material is one of low mobility inherently, there will always be a gradient in the performance across an InP contact layer device. For the usual dimensions encountered in PIN detector devices (50-100 microns is the usual width of the p.sup.+ layer, and thereby the effective device width), this gradient always impacts the speed of performance. The net result is a portion of the device is found to be not subject to these problems (known as the "sweet-spot"), assumingly not subject to the disparity in carrier transport times.
With the advanced needs in the communications industry, there is an ever increasing need to have detectors that are highly linear in response over a great number of tones or individual modulation signals, in order to minimize distortion of the analog signal. Furthermore, there is needed a detector which is capable of performance at the relatively high power levels that are demanded of detectors in the CATV and other communications industries. Finally, there is a need for high volume manufacturing as the detectors are used greatly in large number applications such as fiber in the loop. There is therefore a need to have passively aligned devices in assembling a detector module, resulting in great accuracy at greatly reduced cost.
A PIN detector for use in the communications industry having increased linearity and increased maximum optical power detection levels without distortion is disclosed herein. To this end, a PIN structure having a high carrier mobility quaternary material cap layer and a ternary photosensitive layer is disclosed that overcomes the limitations of low mobility devices as described above. The quaternary materials have much greater carrier mobility than InP material and thereby a much shorter carrier transit time across these layers. This reduced carrier transit time effect results in a much more linear response and accordingly greatly reduced intermodulation distortion. Furthermore, having a cap layer of quaternary material allows for greater power level detection without distortion than for similar structures with InP material cap layers. This is due to the larger bandgap discontinuity between InP and InGaAs materials, than between suitable InGaAsP material and InGaAs. At high optical power illumination levels, where the number of photogenerated carriers are much higher than at lower power levels, even small bandgap discontinuities can result in reduced frequency response and saturation effects caused by "pileup" of electrical charge at the discontinuity.